Experimental evidence for antagonistic effects of sperm competition in two sea urchin species

Sperm competition, a phenomenon where sperm from multiple males vie for fertilizing eggs, has been extensively studied in species with internal fertilization, yet remains less understood in those with external fertilization. To shed light on this issue, researchers conducted experiments on two sea urchin species: Paracentrotus lividus and Dendraster excentricus.

In these studies, eggs from a single female were exposed to sperm from three different males, either separately (non-competitive treatment) or pooled together from three males (competitive treatment). Fertilization success, measured as the proportion of eggs fertilized within two hours, was compared between the two treatment groups.

Additionally, sperm motility, assessed by the percentage of motile sperm and curvilinear velocity, was also evaluated. For Paracentrotus lividus, the researchers determined the sperm concentration at which fertilization success reached 50% of the maximum observed value (F50) and calculated the fertilization efficiency (Fe) of the sperm, representing the probability of a sperm-egg encounter leading to successful fertilization.

In Dendraster excentricus, fertilization success was directly measured at a sperm concentration expected to achieve 50% fertilization. The findings revealed that sperm competition led to reduced fertilization success and increased F50 in both species, indicating antagonistic effects that could potentially compromise fertilization under sperm-limited conditions in external fertilizers.